194 
MR.G. GORE -m HYDRO ] ’ J ,TJ ORIC ACID. 
also gave 4-08 per cent, of yellow sulphur (containing arsenic) by decomposing the sul- 
phurous acid in it by an excess of sulphuretted hydrogen. A second sample, sent to 
me by the same makers, had a specific gravity of 1-8.10, and contained 4- 8 per cent, of 
H 2 S0 4 . A third sample, made near Birmingham, of specific gravity 1-217, contained 
3‘27 per cent, of H 2 S0 4 and .2-16 per cent, of ignited fixed matter precipitated by 
ammonia, including 0-76 per cent, of silica= 1-824 per cent, of 2HF, SiF 4 , A fourth 
sample, obtained from Wolverhampton, of specific gravity 1-1097, contained 0-55 per- 
cent. of H 2 S0 4 . A fifth, obtained from London, of specific gravity 1-1894, contained 
10-54 per cent, of H 2 S0 4 . A sixth, from Gorlitz, in Prussia, of specific gravity 1-1741, 
contained only traces of sulphuric acid. A seventh, from Mr. Hornby, of Sheffield, of 
specific gravity 1-188, contained traces only of sulphuric, hydrochloric, or hydrofluosilicic 
acids, or of fixed matter. And an eighth sample, from Mr. William Collier, Sheffield, 
of specific gravity 1'248, contained no sulphuric or hydrochloric acid, and left only a 
trace of solid residue on evaporation to dryness: by neutralization with ammonia, 1*70 
per cent, of fixed matter was obtained after washing and ignition ; this fixed matter 
contained 1'46 part of Si 0 2 =3-5 per cent, of 2HF, SiF 4 in the acid; the fixed matter 
also contained iron, &c. A second sample, weighing twelve pounds, from the same 
maker gave 723 grains of sulphur by treatment with excess of sulphuretted hydrogen. 
All the samples contained large but variable amounts of sulphurous acid, and gave 
copious deposits of sulphur &c. by treatment with excess of sulphuretted hydrogen ; 
they also contained very variable percentages of HF, varying from about 10 to 70. Oil 
of vitriol is sometimes added to weak hydrofluoric acid to enable it to etch glass in a 
suitable manner ; it imparts to it a fictitious strength, probably by abstraction of. water, 
A sixty per cent, acid requires dilution before it can be used for etching glass; too 
strong an acid etches glass roughly. An acid of 70 per cent., and containing 5-76 per 
cent, of H 2 S0 4 , instantly converted filtering-paper into a jelly at 60° Fahr. ; it was too 
strong for retention in gutta-percha bottles. 
A small gutta-percha jug of the annexed form was sometimes employed for Fig- 14- 
taking the specific gravities of samples of weak acid. The neck of the vessel was 
less than half an inch internal diameter, and was closed by placing a gutta-percha 
ball with a handle loosely in it. Any excess of liquid above the ball was absorbed 
by application of sponge previous to weighing. 
Sulphurous acid is frequently present in commercial hydrofluoric acid to such an 
extent as to be readily detected by the odour ; it may be easily found by adding shavings 
of metallic tin to the acid. Tin was found to be much better than zinc ; it produced a 
cloud of sulphur at once, and evolved sulphuretted hydrogen readily. Bifluoride of tin 
also evolved sulphuretted hydrogen easily. Sulphuretted hydrogen gas in excess was 
passed through commercial hydrofluoric acid ; some time after the commencement of its 
passage free sulphur appeared, and then accumulated quickly. No increase of sulphuric 
acid was found in the liquid after this reaction. If the reaction is correctly represented 
by the following equation, 2FI 2 S-{-S0 2 =2H 2 0-{- 3S, then 96 parts by weight of sulphur 
